The present invention relates to a modular sub-unit for the production of crystals in a suspension crystallization system. The present invention also relates to a suspension crystallization system comprising said sub-unit and a suspension crystallization process using said modular sub-unit or said suspension crystallization system.
Suspension crystallization and crystallizers find many applications in the purification and concentration of components of mixtures and other compositions, particularly in the food (freeze concentration) and the chemical industries, including waste water treatment.
The largest cost factor in suspension crystallization is the crystallizer, which has an inner wall on which scrapers are running. Small suspension crystallizers have an expensive, finely machined inner wall optimized for scraping, which allows for high heat-exchange loads per square meter of scraped surface.
Larger suspension crystallizers and higher throughput are technically quite challenging because it is difficult to maintain a constant and uniform contact of the scraper with large surface area walls. Therefore the scraping of the wall for large crystallizers is often not sufficient because for economical/technical reasons it is not possible to machine the large surface area wall. Achieving a uniform touching of the scraper would require precise machining using custom-built special large machining devices. This makes such large crystallizers unsuitable or too expensive for use in applications having a high concentration of crystallizing component in the mother liquor because under these conditions crystals are strongly attached to the wall. If not perfectly scraped, a crystal layer will form on the wall leading to knife damage, high nucleation rates and a loss of heat transfer and thus capacity. However interest in the field of end purification of chemicals is to go to higher concentration of crystallizing component in the mother liquor, which causes crystals to adhere firmly to the wall, and therefore a uniform scraping is extremely important. WO 2008/113386 discloses a crystallizer for simultaneous crystallization of salt and water in an aqueous salt solution consisting of several adjacent crystallization modules arranged to provide a large scraped cooled surface. This crystallizer is provided with two separate slurry outlets, one at the top for ice crystals and one at the bottom for salt crystals, and therefore this crystallizer is specifically designed for use with aqueous systems only. Although the disclosed crystallizer allegedly has a large scraped cooled surface and relatively high heat transfer capacity, it suffers from problems associated with poor mixing, stagnant zones and crystal agglomeration. Additionally one skilled in the art will recognize that uniform scraping of the disclosed coaxial cooled surfaces within a module is challenging due to the complicated arrangement of the cooled surfaces. In large crystallizers, including the above disclosed crystallizer, mixing of the suspension layer at the wall, where heat transfer occurs and new crystals are formed, is poor resulting in a buildup of crystals around the scraper construction. Conventionally, as for example, disclosed in EP1398064B1, U.S. Pat. Nos. 3,283,522, and 4,316,368, the problem of poor mixing at the wall is addressed by means of expensive and mechanically complex designs having high maintenance and operation costs. One example is a system having a counter-rotating coaxial mixer and scrapers and independent dual drive systems, one for the scrapers and one for the mixer. Equally complex and expensive are constructions based on a draft tube and mixer. Employing either of these designs cause the manufacturing and maintenance costs to rise steeply.
Some example suspension crystallizers consisting of a crystallization section and a mixing section are known, such as those disclosed in WO 2008/155640 A1 or U.S. Pat. No. 6,241,954. However when such crystallizers are built on a large scale, they suffer from the above described problems such as lack of efficient mixing and effective scraping.
The above-discussed problems related to poor mixing and crystal agglomeration may often result in further problems of crystal blockage inside the crystallizer. Dealing with such blockages requires regular defrosting, which is disadvantageously time consuming and requires operator attention.
Therefore, it would be useful to have an improved design for suspension crystallization systems, particular larger and higher throughput ones, that have improved mixing and wall scraping properties without the mechanical, operational and maintenance complexity and costs associated with the prior art systems. Additionally it would be desirable to have a crystallizer design solution that would simplify the manufacture of a variety of crystallizer sizes, all having favorable mixing and wall scraping performance. Also desirable would be to have a simple suspension crystallization process making use of such systems and their sub-units without requiring extensive defrost cycles (periodic heating of the crystallizer to melt crystal agglomerates that have formed during operation) and maintenance and/or operational complexity.